Low picogram levels of the E series prostaglandins, PGE1, PGE2, 19-hydroxy PGE1 and 19-hydroxy PGE2, in human semen were analysed by silica capillary column gas chromatography with electron-capture detection after conversion to the methyl ester O-trimethylsilyl derivatives of the corresponding B series prostaglandins. The method was used to detect traces of semen on post-coital vaginal swabs, and on rectal, oral and skin swabs after simulated sexual acts. Semen was detectable on a vaginal swab taken 58 h after intercourse, and was readily detectable for at least 6 h on rectal and skin swabs. Preliminary results suggest that the ratios of prostaglandins on vaginal swabs may indicate how recently intercourse occurred.